The present invention relates to an antenna element and, more particularly, to a broadband antenna element incorporated in a portable electronic device main unit or a module, or the like, connected to an extension terminal (an optional terminal) of a portable electronic device.
A UWB (Ultra-Wide Band) technique represents ultra wide band wireless communication as its name signifies. UWB is defined as a wireless technique that occupies 25% or more of a center frequency or a bandwidth of 1.5 GHz or more. In a word, the UWB technique is a technique that establishes communication by use of a short pulse in an ultra wide band (1 ns or less in normal times), thereby effecting a revolution in wireless transmission.
A definitive difference between the related-art wireless technique and the UWB technique can be said to lie in presence/absence of a carrier wave. Under the related-art wireless technique, a sinusoidal wave of a certain frequency, which is called a carrier wave, is modulated by means of various methods, and data are transmitted or received. In contrast, the carrier wave is not used under the UWB technique. The UWB technique uses a short pulse in an ultra wide band as described in connection with the definition of the UWB technique.
The UWB technique uses an ultra wide frequency band, as its name signifies. In the meantime, the related-art wireless technique uses only a narrow frequency band. This is because a radio wave can be more utilized as the frequency band becomes narrower. The radio wave is a finite resource. The reason why the UWB technique gains attention despite its ultra wide band is output energy achieved at respective frequencies. Although the UWB technique uses a wide frequency band, outputs achieved at respective frequencies are very small. Since outputs achieved under the UWB technique are buried in noise, the UWB technique can be said to have the very low potential of causing interference with another wireless communication. The FCC (Federal Communications Commission) in US has granted permission for UWB transmission because UWB transmission can be performed in a legally-prescribed manner within a range from 3.1 GHz to 10.6 GHz at a limited transmission output of −4.1 bBmMHz.
An antenna basically utilizes a resonance phenomenon. A resonance frequency of an antenna is determined by a length of the antenna. However, in an UWB including many frequency components, it is difficult to cause the antenna to perform resonance in an UWB. Consequently, the wider the frequency band of a radio wave desired to be transmitted becomes, designing of the antenna becomes correspondingly harder.
Taiyo Yuden Co., Ltd. has successfully developed an ultra-small ceramic chip antenna measuring 10 mm×8 mm×1 mm for UWB. Since the UWB technique is opened for FCC commercial applications, the technique attracts an attention as the next-generation short-range wireless communication standards. For one thing, there is a chance of simultaneous realization of high-capacity data transmission and low power consumption. For another thing, under the UWB technique, occurrence of interruption is avoided by transmission of a very-low output pulse that is equal to or less than a transmission noise threshold value. By means of development of the antenna, the UWB broadens the duty of a wireless industry from military applications to fields of commercial applications where data belonging to digital devices, such as a PDP (Plasma Display Panel) TV and a digital camera, are linked together at extremely high speed.
Such an UWB antenna can be used for applications, such as Bluetooth® and a wireless LAN (Local Area Network).
The Bluetooth® is an open standard for the advanced technology that implements wireless communication of audio and data within a comparatively-small range among a desktop computer, a notebook computer, a hand-held device, a PDA (Personal Digital Assistant), a portable cellular phone, a printer, a scanner, a digital camera, and a mouse of a computer. Since communication is performed by use of a radio wave in a 2.4 GHz band, which is available everywhere on Earth, under the Bluetooth wireless technique, the technique can be utilized in the world. To put it briefly, the Bluetooth technique obviates the necessity of a cable used for connection of digital peripheral devices, and inconvenience related to cable connection is now a thing of the past.
The word “wireless LAN” signifies a LAN utilizing a transmission channel other than a wired cable, such as a radio wave and infrared radiation.
Various broadband antenna devices have hitherto been known in this field. For instance, Patent Document 1 (JP-A-2003-273638) discloses a broadband antenna device that is tuned for a target frequency characteristic and that can diminish interference originating from unwanted frequency bands and interference to frequency bands other than a target frequency band. According to Patent Document 1, the broadband antenna device has a plane conductor bottom plate and a plane radiation conductor that is used while standing on the surface of the plane conductor bottom plate in a direction crossing the plane conductor bottom plate. A feeding point is positioned on an outer periphery of the plane radiation conductor or its neighborhood. The plane radiation conductor is provided with one or more cutouts that are formed by cutting a portion(s) of the plane radiation conductor.
Patent Document 2 (JP-A-2003-283233) discloses a compact broadband antenna device that addresses problems, such as cost, the purpose of use, and incorporation of an antenna device into equipment and that enables a reduction in manufacturing cost and covers a wide frequency range. According to Patent Document 2, the broadband antenna device has a plane conductor bottom plate and a polygonal plane radiation conductor that is used while standing on the surface of the plane conductor bottom plate in a direction crossing the plane conductor bottom plate. An apex of the polygonal plane radiation conductor is taken as a feeding point.
Patent Document 3 (JP-A-2003-304114) discloses a broadband antenna device that uses a plane radiation conductor as a radiation conductor and that can be miniaturized to a much greater extent. According to Patent Document 3, the broadband antenna device has a plane conductor bottom plate and a plane radiation conductor that is arranged on a surface of the plane conductor bottom plate so as to stand in a direction crossing the plane conductor bottom plate. The plane radiation conductor has a plurality of conductor portions arranged in a direction crossing the plane conductor bottom plate when remaining in an upright position on the surface of the plane conductor bottom plate. The plurality of conductor portions is interconnected by means of a low-conductivity member whose conductivity approximately ranges from 0.1 [/Ωm] to 10.0 [/Ωm].
Patent Document 4 (JP-A-2003-304115) discloses a lower-profile broadband antenna device. According to Patent Document 4, the broadband antenna device has a conductor bottom plate and a radiation conductor, they are connected to each other by means of a feeder line for feeding power and they are arranged in such a way that at least portions of the bottom plate and the conductor oppose each other. A substance whose conductivity achieved in a usable wireless frequency approximately ranges from 0.1 [/Ωm] to 10.0 [/Ωm] is placed at a position where the conductor bottom plate and the radiation conductor face each other.
An antenna device for UWB purpose that enables broadening of a frequency band and enhancement of a frequency characteristic is proposed in Patent Document 5 (JP-A-2005-94437). According to Patent Document 5, the UWB antenna device has a radiation element made up of an upper dielectric substance, a lower dielectric substance, and a conductor pattern sandwiched therebetween. The conductor pattern has a feeding point provided in an essential center of a front surface. The conductor pattern is built from an inverse triangular section, which has a right tapered portion extending from the feeding point to a right side surface at a predetermined angle and a left tapered portion extending from the feeding point to a left side surface at a predetermined angle, and a rectangular portion whose bottom edge contacts an upper edge of the inverse triangular portion. A ground plate, extending within a plane identical with the conductor pattern (a radiation element), is electrically connected to the feeding point of the conductor pattern.
Various thin antennas for UWB purpose that cover frequencies of a UWB from 3.1 GHz to 10.6 GHz have been proposed. For instance, a broadband oval ring antenna whose radiation element is made in an oval shape has been known (see; for instance, Non-Patent Document 1). Further, a broadband oval ring antenna for which an attempt has been made to miniaturize a ground plate has also been known (see; for instance, Non-Patent Document 2). A broadband oval ring antenna whose gain is improved by 9 GHz or more has also been known (see; for instance, Non-Patent Document 3).    [Patent Document 1] JP-A-2003-273638    [Patent Document 2] JP-A-2003-283233    [Patent Document 3] JP-A-2003-304114    [Patent Document 4] JP-A-2003-304115    [Patent Document 5] JP-A-2005-94437    [Non-Patent Document 1] Hattori, Kondo, Yamauchi, Nakano, “Broadband Oval Ring Antenna,” General Assembly of IEICE, B-1-104, Osaka, March in 2005.    [Non-Patent Document 2] Hattori, Yamauchi, Nakano, “Broadband Oval Ring Antenna Second Report,” Society Assembly of IEICE, B-1-82, Hokkaido, September in 2005.    [Non-Patent Document 3] Hattori, Yamauchi, Nakano, “Broadband Oval Ring Antenna Third Report,” General Assembly of IEICE, B-1-165, Tokyo (Kokushikan University), March in 2006.
In the broadband antenna devices disclosed in Patent Documents 1 through 3, the plane radiation conductor stands on the surface of the plane conductor bottom plate in the direction crossing the plane conductor bottom plate. Therefore, the profile of the broadband antenna device becomes great.
In the meantime, the broadband antenna device disclosed in Patent Document 4 presents a problem of a narrow operable band. The antenna for UWB purpose described in Patent Document 5 also presents a problem of a narrow operable frequency band ranging from about 4 GHz to 9 GHz.
The broadband oval ring antennas described in Non-Patent Documents 1 through 3 cover the UWB ranging from 3.1 GHz to 10.6 GHz. However, in such a broadband oval ring antenna, the height of the antenna determines an operating frequency; hence, the height requires about a (quarter) wavelength (about 24 mm) of 3.1 GHz. In order to reduce the height, miniaturizing an antenna by use of a high dielectric substance (ceramic, or the like) is conceivable. However, even when an attempt is made to pursue miniaturization as mentioned above, the height comes to about 10 mm in many cases.